The present invention relates to a sphere grinding apparatus for grinding spheres, and more particularly to a sphere grinding apparatus for grinding spheres for use in a rolling bearing, for example, ball bearing or the like. In addition, the present invention also relates to a sphere grinding method.
A conventional sphere grinding apparatus holds spheres between two grinding discs consisting of a rotational disc and a stationary disc to grind the spheres. The structure is formed such that only the rotational grinding disc (hereinafter called a "rotational disc") is rotatively borne. The stationary grinding disc (hereinafter called a "stationary disc") is connected to a rod of a hydraulic cylinder.
For example, an example of the sphere grinding apparatus has been disclosed in Japanese Patent Utility-Model Unexamined Publication No. 54-164189. As shown in FIG. 1, the sphere grinding apparatus 140 incorporates a rotational disc 105 and a stationary disc 106 disposed opposite to each other so that their central lines are made to be horizontal. Also in Japanese Patent Utility-Model Unexamined Publication No. 47-8599, an example of the sphere grinding apparatus has been disclosed. As shown in FIG. 2, the sphere grinding apparatus 150 incorporates a rotational disc 105 and a stationary disc 106 disposed opposite to each other so that their central lines are made to be vertical.
In each of the sphere grinding apparatuss 140 and 150, supply, discharge and circulation of the spheres between the rotational disc 105 and the stationary disc 106 are performed by an arbitrary means. When the central lines of the rotational disc 105 and the stationary disc 106 are made to be horizontal, that is, supply, discharge and circulation of the spheres in the sphere grinding apparatus 140 shown in FIG. 1 are performed by a method, for example, as shown in FIG. 3 which is an enlarged perspective view showing an essential portion.
In the grinding process, it is impossible to avoid a heat generation at a grinding portion, a rotationsl spindle, the another parts and so on. These heat generation may cause non-uniformed thermal expansion of the grinding apparatus. Particularly, since the sphere grinding apparatuss 140 and 150 shown in FIGS. 1 and 2 have the structure that the rotational disc 105 and the stationary disc 106 are supported such that the surfaces opposite to the facing surfaces of the two discs are supported, it is impossible to avoid such a heat generation in view of a structural design thereof. Therefore, influence of heat produced in a rotational spindle 103 and rise in the temperature of grinding solution are exerted. As a result, deviation in the centers of sphere guide grooves of the two discs and that in the parallelness of the opposite surfaces of the two discs take place. Therefore, there arises a problem in performing an operation for grinding spheres. That is, the sphericity of the produced spheres and the machining accuracy including roughness deteriorate.
Conventionally, some of idea for refraining the heat generation are suggested, for example, the use of the low thermal expansion material, the use of a bed which has a relatively small temperature raising characteristic and high rigidity, the use of compulsive cooling mechanism, and so on. However, such idea is accompanied with a remarkable design change and its corresponding cost-up. Further, there is a limit for completely preventing the deviation from generating.
To solve the above-mentioned problem, a sphere grinding apparatus has been disclosed in Japanese Patent Unexamined Publication No. 5-57602. As shown in FIG. 4, the sphere grinding apparatus 160 incorporates a central shaft 103 having a common central line; a sleeve 107 disposed to surround the central shaft 103 and permitted to be rotated; and a housing 104 for rotatively supporting the sleeve 107. A stationary disc 106 is joined to the central shaft 103, while a rotational disc 105 is joined to the sleeve 107. Thus, a dual structure is formed.
Since the sphere grinding apparatus 160 has the dual structure, the central shaft 103 and the sleeve 107 can be expanded such that the coaxial relationship is maintained even if heat is produced in the rotational spindle or the temperature of the grinding solution is raised. Therefore, deviation in the centers of the sphere guide grooves of the two discs and that in the parallelness of the opposite surfaces of the two discs do not easily take place. Thus, the problems of deterioration in the sphericity and the machining accuracy including the roughness which arise when spheres are ground can be prevented. However, errors in the rotational accuracy between the housing 104 and the sleeve 107 and errors in the rotational accuracy between the sleeve 107 and the central shaft 103 are accumulated. Thus, the errors becomes critical. Moreover, a large number of elements is required and assembly and adjustment become too complicated. As a result, there arises a problem in that the cost of the grinding apparatus cannot be reduced.
Although the above-mentioned structure does not sometimes arise any problem when the spheres are accommodated in a bearing for use in a conventional hard disk drive (HDD), adaptation to HDD, CD-ROM or other units having large capacities becomes unsatisfactory. For example, severe evaluation criteria are se t for runout of a nonrotational synchronous component (NRRO) Also a severe requirement has been made for the sphericity of the produced spheres and the machining accuracy including the roughness.